遇到了这样的错误：
Undefined variable: conn on line 33
Fatal error: Uncaught Error: Call to a member function close() on null
代码如下
<link rel="stylesheet" href="course.css">
<?php
$title = "College";
include('include/header.inc');
?>
<h1>College</h1>
<?php
include('include/nav.inc');
?>
<?php
if (!empty($_GET['id'])) {
include('include/db_connect.inc');
$name = $_GET['courseid'];
$sql = "select * from CourseDetails where courseid = ?";
$stmt = $conn->prepare($sql);
$stmt->bind_param("i", $name);
$name = $_GET['courseid'];
$stmt->execute();
$result = $stmt->get_result();
if ($result->num_rows > 0) {
while ($row = mysqli_fetch_array($result)) {
print "<h2>{$row['coursecode']}</h2>";
print "<h3>by {$row['name']}</h3>";
print "<h4>{$row['subject']}</h4>";
print "<h5>{$row['instuctors']}</h5>";
print "<h6>{$row['weeks']}</h6>";
print "<p>{$row['description']}</p>";
}
}
}
?>
<?php
$conn->close();
include('include/footer.inc');
?>
